Ternary azeotropic compositions

ABSTRACT

CERTAIN TERNARY MIXUTRES OF TETRACHLORODIFLUOROETHANE (SYM-, ASYM-, AND MIXTURES THEREOF) ISOPROPANOL AND NITROMETHANE ARE USEFUL AS SOLVENTS TO REMOVE ROSIN FLUXES FROM PRINTED CIRCUIT BOARDS CONTAINING THE SAME. THESE MIXTURES ARE USEFUL NOT ONLY BECAUSE OF THEIR HIGH SOLVENCY CHARACTERISTICS BUT ALSO BECAUSE THEY EXHIBIT ESSENTIALLY THE CONSTANT BOILING CHARACTERISTICS OF A TERNARY AZEOTROPE WHICH IS FORMED BETWEEN THESE COMPONENTS, THEREBY FACILITATING HANDLING AND PURIFICATION OF THE SOLVENT MIXUTRES WITHOUT SIGNIFICANTLY ALTERING THEIR COMPOSITIONS. THE TERNARY MIXTURES DISCLOSED HEREIN EXHIBIT SUBSTANTIALLY HIGHER SOLVENCY CHARACTERISTIS FOR ROSIN FLUXES THAN THE PURE SOLVENTS, TETRACHLORODIFLUOROETHANE, ISOPROPANOL OR NITROMETHANE.

United States Patent 3,694,368 TERNARY AZEOTROPIC COMPOSITIONS Oliver A.Barton, Florham Park, N..l., and Kevin P. Murphy, Orchard Park, N.Y.,assignors to Allied Chemical Corporation, New York, N.Y. No Drawing.Filed Feb. 26, 1971, Ser. No. 119,372 Int. Cl. Clld 3/44 US. Cl. 252-4714 Claims ABSTRACT OF THE DISCLOSURE Certain ternary mixtures oftetrachlorodifiuoroethane (sym-, asym-, and mixtures thereof)isopropanol and nitromethane are useful as solvents to remove rosinfluxes from printed circuit boards containing the same. These mixturesare useful not only because of their high solvency characteristics butalso because they exhibit essentially the constant boilingcharacteristics of a ternary azeotrope which is formed between thesecomponents, thereby facilitating handling and purification of thesolvent mixtures without significantly altering their compositions. Theternary mixtures disclosed herein exhibit substantially higher solvencycharacteristics for rosin fluxes than the pure solvents,tetrachlorodifluoroethane, isopropanol or nitromethane.

BACKGROUND OF THE INVENTION The electronic industry has sought forsolvents which can efliciently remove rosin fluxes from printed circuitboards containing the same. The rosin fluxes are intentionally depositedon the surface of the circuit boards prior to soldering on electroniccomponents, but must be removed after soldering in order to achievemaximum reliability of the printed circuits. The solvent must not onlybe highly eifective for removing the undesired rosin flux but must, forcommercial applications, be stable, and inert togards the electroniccomponents on the circuit board itse A variety of solvents have beentested for such purposes but generally have been found to be lacking, toa greater or lesser extent, one or more of the above describedproperties. For example, whereas highly chlorinated solvents, such as CHCl and CHCl are highly eifective for the removal of rosin flux; suchsolvents, when used alone, attack the electronic components on thecircuit board. Such solvents also require the addition of a stabilizerto prevent decomposition. A variety of non-constant boiling mixtureshave been employed to achieve the desired solvency, while retaining thedesired inertness towards the electrical components. Preferentialevaporation of the more volatile component of such mixtures, however,can result in mixtures having less desirable properties, such as lowersolvency for rosin fluxes, less inertness towards the electricalcomponents and increased flammability.

A number of binary azeotropic (constant boiling) mixtures have beenemployed for the purpose of cleaning electrical circuits, which aifordmany of the advantages obtainable with solvent mixtures, but which donot sufier from the above described major disadvantage possessed bynon-constant boiling solvent mixtures. Illustrative of such binaryazeotropic systems are the azeotrope of 1,1,2-

3,694,368 Patented Sept. 26, 1972 trichlorotrifluoroethane and methylenechloride, B.P. 37 C. at 760 mm. pressure (U.S.P. 2,999,817) and thebinary azeotrope of 1,l,2-trichlorotrifluoroethane and methyl alcohol,B.P. 39 C. at 760 mm. pressure (U.S.P. 2,999,- 816). Unfortunately,however, the solvencies of these binary azeotropi-c compositions for thecommon rosin fluxes which are employed in the manufacture of printedcircuits, are not as high as might be desired and the solvents eitherleave deposits on the boards or become cloudy after use.

SUMMARY OF THE INVENTION It is accordingly the major object of thisinvention to provide constant boiling or essentially constant boilingcompositions which, while meeting the requirements of inertness toelectronic components; posses particularly high solvency towards theresin fluxes which are commonly used in the manufacture of printedelectrical circuits and which therefore permit repeated cleaningtreatments of such electrical circuits without significant redepositionof the flux upon the surface of the printed circuits upon removal of thesolvent by evaporation.

Another object of the invention is to provide novel, constant boiling oressentially constant boiling compositions of matter which do notsubstantially change composition upon evaporation.

The present invention is directed to constant boiling compositions, andparticularly to a ternary azeotropic composition, and equivalentmixtures, which possess enhanced solvency for polymeric compounds,particularly rosin solder fluxes used in the manufacture of printedcircuit boards. The novel ternary azeotropic composition is a constantboiling mixture which contains about 67 weight percent oftetrachlorodifluoroethane, about 22.2 weight percent of isopropanol andabout 10.8 weight percent of nitromethane. The composition of thismixture may be varied somewhat without altering the essential azeotropicnature or constant boiling characteristic of the same, or the solvencycharacteristics of the same.

It has also been found that this azeotropic mixture is inert toelectronic components on printed circuit boards and that it exhibitsunusually high solvency towards rosin fluxes which are commonly employedin the manufacture of such boards. As a consequence, samples of thenovel azeotropic composition may be employed repeatedly and effectivelyas solvents to remove such rosin fluxes and upon evaporation leave noperceptible residues. Moreover, the solvent solutions remain clear evenafter repeated use. Since the azeotrope boils at a constant temperature,evaporation or distillation of the azeotrope in whole or in part doesnot change the composition of the liquid mixture. This'is significantsince it enables the azeotropic mixture to be handled and purifiedwithout the adverse eifect of having its composition change as wouldoccur with a non-azeotropic mixture.

The tetrachlorodifluoroethane component is available commercially. Thismaterial, as sold commercially, is a mixture of the symmertical isomer,1,1,2,2-tetrachloro-l, Z-difluoroethane, and the asymmetrical isomer,1,1,l,2- tetrachloro-2,2-difluoroethane, in the ratio of approximately69 mol percent and 31 mol percent, respectively. We have found thateither the pure isomers or mixtures thereof form minimum boilingazeotropes with isopropa- 1101 and nitromethane having approximately thesame composition, boiling point, and properties. Accordingly ourinvention includes the ternary zeotrope derived from either thesymmetrical or asymmetrical tetrachlorodifluo- 4 of pine tree gum,containing abietic acid and related substances) was heated at 260 for 24seconds. The cured rosin solder illux was cooled and then ground to afine powder. Five parts of a test solvent was heated to 75 the usualtemperature of vapor cleaning baths, and

product is said to contain as a major ingredient some form roethane ormixtures thereof. A suitable grade for the 5 mixture of isomers oftetrachlorodifluoroethane is sold agitated at that temperature whilesmall increments of by Allied Chemical under the trademark Genetron 112.the powdered cured rosin solder flux were added to the Further referencein this specification and the claims aphot test solvent. The solvencyendpoint of the test was pended hereto, unless otherwise indicated, totetrachloreached when a persistent cloud of insolubles wasatrodifluoroethane is intended to include the pure isomers tained.

and/ or any mixtures thereof. In order to compare the solvency power ofthe ternary In the following examples parts and percentages areazeotrope of our invention with the individual comby weight andtemperatures are given in degrees centiponents thereof the solubility ofthe cured rosin solder grade, unless otherwise specified. flux wasdetermined in each of these solvents and the azeotropic mixture thereof,by the above described test EXAMPLE 1 method. The results obtained wereas follows: Preparatlon of ternary azeotrope Wt. percent (A) A mixtureof equimolecular proportions of tetra- Tetrachlorodifluoroethane 0.74chlorodifiuoroethane (the commercial mixture of th Isopropanol 7.5isomers, B.P. 92.8"), isopropanol (B.P. 820), and ni o- Nitromethane 1.3methane (B.P. 101.5), was charged to a still and heated Azeotrope 9.7therein to reflux. The mass was then distilled. A constant Th boilingfraction at about 74 was collected and found to 656 results mdlcate thesurpnsmg.greater. q i contain all three components. This fraction wasredistilled Power of thehazeotrope as compared wlth the mdwldual andanalyzed by gas chromatography. The composition componentst -M Y the ofthe azeotrope was found to be: azeotrope for the rosin solder flux 18more than twice that Percent which would be expected from the sum of thesolvency Tetrachlorodifluoroethane 670 power of the ind v dualcomponents indicating the preslsopropanol 222 ence of a synergisticsolvency characteristic in the azeo- Nitromethane 108 twp: mlxtum'EXAMPLE 3 The boiling point of this azeotrope was 74.6" at 760 mm.pressure. To simulate actual manufacturing practice and to deter- (B)The procedure of part A above was repeated usmine the precise rate ofremoval of rosin solder flux from ing in place of the commerciallyavailable mixture of an inert substrate, aluminum sheets, 6" x 1 x 2",were tetrachlorodifluoroethane isomers an equivalent amount coated witha commercially available rosin solder flux of the pure (97 mol percent)asymmetrical isomer. A and the coated sheets were heated in an oven at204.4 ternary azeotrope of essentially the same percentage comfor fiveminutes. After cooling, the coated sheets were exposition was obtained.The boiling point of this azeotrope posed to refluxing vapors of varioussolvents and the was 74.7 at 760 mm. pressure. 40 length of timerequired to clean the coated sheets deter- (C) The procedure of part Aabove was repeated using mined. The results of these tests are set outin the followin place of the mixture of tetrachlorodifluoroethane isoingtable.

TABLE Wt. of Time of Reflux flux in removal, Solvent temp., gramsseconds Description of substrate Nitromethane 101. 2 0. 0267 240 Plainlyvisible Wax-like film over much of sheet. 1.2% by wt. of coatingremaining Isopropanol 82.0 0.0250 100 Clean. Tetrachlorodifluoroethane..92. 8 0. 0243 120 Do. Azeotrope 76. 4 0. 0250 18 Do.

mers, an equivalent amount of the pure (99 mol percent) These dataclearly indicate the superior solvency power symmetrical isomer. Aternary azeotrope of essentially of the azeotrope of our inventioncompared with that of the same percentage composition was obtained. Theboilthe individual solvents contained in the azeotropic mixing point ofthis azeotrope was 74.2 at 760 mm. pressure. ture. They clearlyindicate, also, the synergistic solvent effect of the azeotrope withrespect to the dissolution of EXAMPLE 2 rosin solder flux.

In the manufacture of printed circuit boards, the de- The azeotropes,being constantboiling, can be efficienfly sired electrical circuit isetched on a copper clad phenolic hahdled at the temperature of the bathWithout the P f or epoxy resin laminated board and the laminate iscoated el'lflal 1058 Of One compOnent y not ffactlonwith a rosin solderflux. After the coating has been dried, Non'aleotmpic Solvent miXthfeS,because of 105565 the board is passed through a molten solder bathwherein of 0116 more Of the Pompohehts 0f the Solvent the circuit jointsare contacted by the molten metal and ture y evaporation durlng e, cannot be reclaimed soldering of the joints is accomplished. After cooling,the tact y collyehtlohal fectificahon- @Xcess rosin fl remaining on theboard must be d, The novel azeotropes of the invention find othersolvent since if such be present in the final assembly, it will lead pphsuch "3 for removing gases and Oils from a to corrosion, poor electricalresistance and other adverse varlety 0f hldusmal items, for the Cleaningof P ff t graphic films and prints, for the removal of buffing com- Inorder to demonstrate the elfectiveness of the azeo- Pounds, h as rouge,and also y h used as heat tropes of our invention as solvents for rosinsolder fluxes change hlbdla, electrical transfe? medla, Chemicalreacused in the preparation of printed circuit boards, the fol- @011mPCha, and as hydraulic lowing test was carried o t, It Wlll be apparentto those skilled in the art that for An amount of a commerciallyavailable rosin solder Specialized Purposes, Various additives could berfi K t N 1544 (th trade designation f a o i porated with the novelsolvent mixtures of the invention, solder flux available from KesterSolder Company. This for ex mp lubricants, detergents, and the like-These additives are chosen so as not to adversely alter the essentialproperties of the mixture for a given application.

The invention is not intended to be limited by any specific embodimentsdisclosed herein but only by the scope of the following claims.

We claim:

1. A constant boiling mixture of tetrachlorodifluoroethane, isopropanoland nitromethane which when distilled at a pressure of 760 mm. consistsessentially of about 67 weight percent of tetrachlorodifluoroethaneabout 22.2 weight percent of isopropanol and about 10.8 weight percentof nitromethane.

2. A mixture as defined in claim 1 in which thetetrachlorodifluoroethane is a mixture of about 69 mol percent of1,1,2,2-tetrachloro-l,Z-difluoroethane and about 31 mol percent of1,1,1,2-tetrachloro-Z,2-difluoroethane.

3. A mixture as defined in claim 1, in which the tetra- References CitedUNITED STATES PATENTS 4/1963 Kvalnes 252171 9/ 1970 Clark et a1 252DIG.9

L'EON D. ROSDOL, Primary Examiner W. E. SCULZ, Assistant Examiner US.Cl. X.R. 252DIG. 9

